Lightning Protection for 12V, 500mA system protection

We have a petrol pump monitoring system which is frequently struck by lightning. It comprises of a 2 wire feed from the control box of the petrol pump sitting a few meters away.

The cables running from the petrol pump to the controller box are placed in the ground inside a plastic sleeve - therefore exposed to the elements. We find that the controller box is frequently damaged by lightning surges.

Can you suggest a simple method to protect this 2 wire system. This pair of course is carrying 500mA of voltage within itself signalling protocol. We were considering to install fuses because they are very simple to replace if damaged BUT what would you suggest?

The Petrol Pumps are mounted below and above ground. Pipes run from petrol tanks below ground to the dispensing units from where a hose is used to fill up the cars. We suspect that they are "ground shocks" from near misses or from a large cell phone mast not too far away that attracts the lightning and conducts it to ground.

The controller units are inside the nearby office where the petrol attendants deposit the cash collected etc.

Lightning can also hit the pumps directly and damage the controller unit this way too.

What your missing in understanding the problem is the reverse nature of electricity in that all of the electrons are in the negative side of the circuit (meaning ground). With lightning, in the last split second before the strike, voltages of up to 1700 volts per centimeter rush through the ground to the point of impact and, what damages equipment is the difference between two (or more) ground points. What you need to do is to have the attendants end completely isolated from ground with a large, shielding conductor connecting it to the pump.

Tx 4 suggestion.
"What you need to do is to have the attendants end completely isolated from ground with a large, shielding conductor connecting it to the pump."
Please elaborate and perhaps you can direct us to the product we may consider.

What you want is to have one (and only one) earth ground connection for the entire circuit. Then, any voltage surge will cause everything to float together. When there are two grounds, you wind up dissipating the voltage differential through your equipment.
Its OK to have a protective ground on the AC side of a power supply; but, it should not be connected to the logic ground on the DC side which can be checked with a multimeter.

After a point, it is impossible to protect things from a direct strike; but, I think your primary issue is two ground points and the resulting voltage difference of nearby strikes. The pump is definitely one ground point and I'll bet there is a second ground where the AC enters the station. If you move the AC ground stake as close as you can to the run between the pump and the station and run a wire from it to the pump, you'll virtually eliminate the differential which will stop the failures. 4 gauge aluminum wire should work fine.

You could use 2 Coaxial cables for the 2 wire feed from the control box to the petrol pump.
The outer conductive sheath can then be used to conduct an earth from the control box to the petrol pump and then to a grounding rod.
While the inner wires supply the +/- DC supply.

"We find that the controller box is frequently damaged by lightning surges."

This is likely due to power surges passing through the 12V, 500mA supply so a DC 12V Lightning Proof Power Surge Protector should solve the problem.

Take a look at surge units from Ditek, specifically the DTK-2MHLP model for the low voltage circuit. This model comes in a number of different voltage specifications so be sure to get the 12 volt model.

This needs to be installed at the controller box end and it does need to be connected to ground. Preferably connected to the same ground as the pump power.

If you have multiple grounds they should be 'bonded' - this means you need a conductive wire that ties the grounds directly together and will prevent the issue mentioned about a difference in grounds. This is to bring all bonded grounds to the same level so you do not have flow induced between them.

In addition to the protector on the 12 volt circuit I would also recommend using a protector on the fuel pump circuit. Probably this DTK-120GP or the 240 volt unit depending on what the pump runs on,

Again this needs to be on the pump's feed line and needs to be grounded. It would be best to be tied into the AC source ground but there may be specific rules for grounding because this is for a fuel pump. Likely the tank has its own ground and the AC source ground is bonded to it.

I am assuming the control box is being powered through the same line that feeds the pump.

The whole point of using a protector at the pump and at the controller is that each protector will cut down on any surge. So if you get a surge on the AC feed it hits the first protector on the pump, then any surge that manages to get through will hit the protector on the DC control and hopefully this prevents problems.

The big problem with any in-ground cables is that lighting anywhere close by can induce a surge. Anything within a quarter mile or so could cause a problem. You should probably look into how well grounded you are now. Any corroded ground connections need to be cleaned up, in some cases there may not be enough grounding and a 2nd or longer rod may need to be put in.

If you were actually having a direct hit there would be obvious physical damage and probably lots of other damaged equipment (if anything else is at this site).

The whole issue with surges and lightning even more so is that nothing is ever guaranteed, you can only do your best effort. Both of the units I suggested are around $50-60 each and maybe be found through electrical suppliers or likely online.

I would recommend you have this done by a professional electrician, especially for the pump power circuit work.

I've established that the 2 wire control system is actually a TTL DC 5 or 12 V system depending on the type of petrol pump used.
Please review answers above in light of this info and suggested protection devices.

Please provide additional info on TTL as I'm not familiar with this protocol.

Just to reiterate - there is a cell phone mast near this particular garage and the claim is that seeing as though this mast would be very well earthed and the petrol pumps perhaps not as well earthed that this could be causing the frequent strikes.
If this is the case please suggest steps to rectify.

Your problem is not that you have poor grounding; it is that you have two good grounds that are several feet apart and, as I said in my first post, are dissipating up to 1700 volts per centimeter times the distance between them.
Wherever the AC mains enter the building is a ground wire running to a stake. The Pump itself is another ground. Measure the distance between the two and do the math.
Move the AC mains ground stake as close as you can to the pump. Make sure the pump and that stake are thoroughly connected to each other, electrically.
That is what will minimize the damage; but, you must understand that nothing can make you lightning proof as the energy is far too high.

DavisMcCarn: It makes a lot of sense - need to investigate this thank you.
What gauge of cable would you suggest to join the 2 earths?
Can we use an inexpensive multimeter in continuity mode or resistance mode to estimate and measure the two earth distance?

nobus: The height is not great - a pretty standard mast height for 3G masts in South Africa - will have to establish the distance.

All you need to measure the distance between the two grounds is a tape measure and the resistance will be so slight as to be unmeasurable.
Fairly common, here in the U.S, is the use of 4 gauge aluminum wire between the ground stake and the AC inlet to the structure; though, if its a long distance 2 gauge or copper would be better.

" if its a long distance 2 gauge or copper would be better." I would have though the longer the distance the thinker the cable recommended to reduce resistance effect?
Pls can you give these diameters in metric.

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